Metal containers used for distribution of consumable beverages must withstand an internal pressure of at least 90 psi to ensure integrity of the can when it reaches the consumer. Such cans are therefore formed from aluminum alloy or steel and are wall ironed to create a relatively thin metal side wall and a thick metal bottom wall having a hollow central portion. The interior of the can is coated with a protective lacquer coating and the bottom end wall is pressed into a final shape between a punch engaging the external surface of the hollow portion of the bottom end and a hollow die entering the can to support an annulus of bottom end material around the hollow portion. Cooperation of the punch and die pulls the end material to conform to the punch and die profiles to create a bottom having an inward countersink zone and an outward conical or radial surface which are connected by a resting radius.
The resistance of the inwardly domed portion to outward bulging is greatly influenced by the size of the resting radius. The smaller the resting radius, generally the higher the possible internal pressure resistance of the can. However, the aforementioned forming process works most advantageously if the resting radius is large. This is to ensure that the metal does not fracture or reduce in thickness when it is pulled around the radius on the forming tools. Unfortunately, however, the larger forming radius reduces the internal pressure characteristics of the can.
It is accordingly one object of the present invention to reduce the resting radius of a beverage metal can bottom to increase the internal bulge strength of the bottom.
Another object of the invention is to reduce the base thickness of metal in the bottom while still maintaining sufficient strength to withstand the product's internal pressure.
Another object is to reform the resting radius of the bottom after cleaning, printing and coating of the can as aforesaid.
A method of reforming the resting radius of a can bottom wall is disclosed in U.S. Pat. No. 4,885,924 which is assigned to Metal Box p.l.c., Reading, England. Therein, a container is rotated by support means while a roll is applied thereto and moved radially inwardly towards the container axis to contact the resting radius. This patented process has at least two disadvantages. First, it requires a separate machine requiring extra controls and conveying which adds complexity and lengthens the can line process. A second disadvantage is that the separate machine is expensive, not to mention the cost of installation and extra conveying equipment.
Another object of the present invention is to reduce the resting radius of can bottoms without necessitating the addition of sequentially performed bottom reforming steps within the can line process.
Another object is to reform can bottoms simultaneously during other can forming steps.
Still another object is to reform the can bottom while simultaneously necking the can open end.
Yet another object is to reform the can bottom without additional conveying.